Just a note. Your topic may suggest Germany has "banned" "water energizers" in general. To make things clear, regulation autorities have made this one product illegal because it didn't comply with current regulations regarding its RF emissions, and this was apparently noticed by radio amateurs.
Regardless of the purpose of a product, this would have happened exactly the same for any other product not complying with regulations.
However voodooish and bullshitty those water energizer gizmos are, I would have been concerned if some country had actually "banned" a whole range of products merely based on the fact that they are bullshit and too expensive for what they are. That may look reasonable, but would open a whole can of worms.
But that's not what happened. It was banned because it just wasn't compliant with current regulations, and was sold with a false declaration of conformity.
The fact buyers are allowed to keep the product - but of course not use it - if they bought it before the ban is the usual procedure in such a case. As long as a given product is harmless when not in use, regulation bodies can't seize people's belongings.
Now that most of those new-agy products are sold with false declarations of conformity would not surprise me one bit. Actually, that's probably the case for a significant fraction of all products on the market when their category allows self-declarations of conformity.